U.S. patent number 4,165,252 [Application Number 05/883,747] was granted by the patent office on 1979-08-21 for method for chemically treating a single side of a workpiece.
This patent grant is currently assigned to Burroughs Corporation. Invention is credited to Stephen R. Gibbs.
United States Patent |
4,165,252 |
Gibbs |
August 21, 1979 |
Method for chemically treating a single side of a workpiece
Abstract
A method for chemically treating a single side of a workpiece,
such as for etching or anodizing a semiconductor wafer, comprising,
placing such a workpiece face down on a flat centrally apertured,
relatively level table having a top or work surface of a size and
shape commensurate with the dimensions of the workpiece and
introducing the liquid for the chemical treatment between the top
surface and side of the workpiece to be treated where the liquid
passes over the entire surface to be treated and then returns to
its source. The method also includes, for certain applications, a
pre-processing of the workpiece by oxidizing the workpiece surface
on the side of the workpiece opposite of the one to be treated to
be treated to prevent creeping of the liquid around the edges
thereof.
Inventors: |
Gibbs; Stephen R. (Escondido,
CA) |
Assignee: |
Burroughs Corporation (Detroit,
MI)
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Family
ID: |
27109995 |
Appl.
No.: |
05/883,747 |
Filed: |
March 6, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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718897 |
Aug 30, 1976 |
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Current U.S.
Class: |
438/694; 205/124;
205/128; 205/148; 438/745 |
Current CPC
Class: |
C25D
11/32 (20130101); C23F 1/08 (20130101) |
Current International
Class: |
C23F
1/08 (20060101); C25D 11/02 (20060101); C25D
11/32 (20060101); H01L 021/312 (); C23F
001/02 () |
Field of
Search: |
;156/636,637,640,654,642,662,345,655,656,657,659
;204/179.1,129.6,15,23,27,29,33,38R,38A |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IBM Technical Disclosure Bulletin, vol. 16, No. 5, Oct. 1973,
Adjustable Fluid Profile Control for Etching by Hecker, p. 1625.
.
Chemical Engineers' Handbook, Third Edition, 1950 (copyright), p.
408 (Rotameters)..
|
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Dwyer; Joseph R. Young; Mervyn L.
Peterson; Kevin R.
Parent Case Text
This is a continuation of application Ser. No. 718,897, filed Aug.
30, 1976, now abandoned.
Claims
What is claimed is:
1. A method of chemically treating a workpiece on one surface only
comprising the steps of:
placing the workpiece with a surface to be treated horizontally
down on a horizontally disposed top surface of a table, said top
surface having a centrally located aperture thereon and the
workpiece and top surface being coextensive:
introducing liquid chemical in an upward direction through the
aperture and across the top surface where said chemical flows
between the top surface and the entire surface to be treated with
sufficient pressure to space the workpiece from the top surface so
that liquid chemical performs the treatment on the entire surface
of the workpiece and at the same time utilizing said introduced
fluid to alone maintain orientation of the workpiece relative to
the top surface during this treatment.
2. The method as claimed in claim 1 wherein said workpiece can be
preprocessed by oxidizing the edges of the side of the workpiece
opposite the surface to be treated by the liquid chemical in the
steps set forth in claim 1.
3. A method of chemically treating an entire surface of a workpiece
comprising the steps of:
horizontally orienting workpiece surface so as to face a
horizontally oriented top surface of a work table, said workpiece
surface and said top surface being coextensive,
placing the workpiece surface in contact with said top surface,
through a centrally located aperture in said top surface
introducing liquid chemicals in an upward direction between said
workpiece surface and said top surface with sufficient pressure to
separate the workpiece surface from the surface to allow said
liquid chemicals to flow over the entire top surface utilizing said
introduced liquid chemicals to alone maintain the orientation of a
workpiece and concurrently chemically treat said workpiece entire
surface.
4. The method as claimed in claim 3 wherein said step of
introducing the liquid chemical takes place centrally of both said
workpiece surface and said top surface and in an upward direction
and flows radially from said place of introduction.
5. The method as claimed in claim 4 wherein the workpiece to be
treated comprises a circular semiconductor wafer and said top
surface is disk shaped.
6. A method of chemically treating an entire surface of a workpiece
comprising the steps of:
horizontally orienting workpiece surface so as to face a
horizontally oriented centrally apertured top surface of a work
table, said workpiece surface and top surface being
coextensive,
placing the workpiece surface in contact with said top surface,
separating said surface and said top surface by the introduction of
upward flowing liquid chemical under pressure between the workpiece
surface whereby the liquid chemical lifts said workpiece and flows
entirely over said top surface and engages the entire area of said
workpiece surface thus concurrently chemically treating said
workpiece surface, and maintaining the orientation of said surface
relative to said top surface by said liquid chemical.
Description
BACKGROUND OF THE INVENTION
This invention relates in general to the chemical treatment of a
workpiece where it is desired to chemically treat only one side of
the workpiece and eliminate the need to process the othre side with
a protective coating.
This invention relates, in particular, to chemically treating, as
by etching or anodizing, a semiconductor wafer on one side only by
a method and apparatus in which the one side of the wafer can be so
treated without the need of providing a protective coating on the
other side so that the latter side will not react to the chemical
treatment as in the case of the presently known methods.
There are many instances when it is desired to perform work on only
one side of the workpiece, as for example, in semiconductor
processing where it is often only necessary to etch or anodize only
one surface of a semiconductor wafer without disturbing the other
surface. Whether the process was etching or anodizing the one
surface, it has heretofore been necessary to coat the opposite
surface with a protective layer to prevent that surface from
reacting with the liquid chemical, and in the case of anodization,
to immerse most of the wafer into the solution with a positive
potential applied to the edge of the wafer, via a clip, and a
negative potential applied to the liquid. In this process not all
the surface to be anodized was utilized since the clip edge of the
wafer must remain out of the solution. Thus, the step of adding the
protective coating on the side not to be treated and the loss of
the portion of the full wafer were extra costs that increased the
ultimate cost of the manufacture of the end product.
Another known prior art method of processing a workpiece, such as a
semiconductor wafer where only one side of the wafer is to be
chemically treated, is to attach vacuum cups or other attaching
means to the other side of the wafer and suspend the side to be
treated into the solution to a depth less than the thickness of the
wafer. This method is also expensive because of the cost of the
attaching means and the difficulties involved in precisely
suspending the wafer into the solution so as not to affect the top
side of the wafer. The only way to protect the top of the wafer in
this method, of course would be to add a protective coating to the
top side where the vacuum cups are attached but this also is an
additional cost even though this process would eliminate the loss
of the area of the wafer where the clip leads were attached in the
process described above.
Another method is to attach the waferback to a suction cup covering
the entire back surface and making electrical contact within the
cup. The fixture is then submerged and only one side is exposed.
However, maintaining a perfect seal to the wafer edge under vacuum
has proven very troublesome.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore a principle object of this invention to provide a
method for chemically processing a single surface of a workpiece in
a simple and inexpensive manner eliminating a number of steps in
the process in the known prior art, thus reducing the cost of the
ultimate end product.
It is more specific object of this invention to chemically treat,
such as etching or anodizing, a semiconductor wafer on one side
without the need for a protective coating on the other side to
protect the latter from the chemical solution.
Another object of this invention is to chemically process a
workpiece such as a semiconductor wafer in which the entire side of
the wafer is processed without the loss of any area of the wafer
due to clipping of the electrode thereto as in the prior art.
This and other objects of this invention are accomplished through
the use of a fixture which has a table with a relatively flat,
relatively horizontal, top surface for supporting the surface of
the workpiece on which work is desired to be performed. The top
surface includes at least one centrally located aperture and
conduit extending from the top surface to a source of liquid
chemical used in processing the workpiece. The workpiece is placed
face down on the table so that the surface to be processed is
facing the table and liquid chemical is then introduced between the
two surfaces through the aperture and allowed to flow over the edge
of the table and over the face of the workpiece back to the liquid
chemical source. When this fixture is utilized to perform
anodization of a semiconductor wafer, electrodes are provided to
supply an electrical potential between the liquid and the wafer.
The electrical contact for the wafer comprises a freely suspended
electrode which will yield to the slight upward movement of the
wafer when the liquid is introduced between the wafer and the
table. Included in this invention is the provision of additional
protection on the opposite surface of the wafer adjacent the
periphery thereof to prevent the creeping of the solution around
the edges of the wafer in certain applications.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional diagrammatic partial view of one
complete table with the top or work surface and a workpiece
supported thereon and showing the means of introducing the liquid
chemical to the surface to be chemically treated;
FIG. 2 is a cross sectional view of the apparatus showing a
plurality of tables for chemically treating a plurality of
workpieces at one time;
FIG. 3 is a cross sectional diagrammatic view of one table for
pre-processing a workpiece for later processing the apparatus of
FIGS. 1 and 2;
FIG. 4 is a top plan view of a workpiece processed in the apparatus
of FIGS. 1-3; and
FIG. 5 is a cross sectional view of the workpiece of FIG. 4 taken
along line 5--5 and looking in the direction of the arrows.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. I, it can be seen that a workpiece 10, shown as a
semiconductor wafer having an upper or top surface 12 and lower or
bottom surface 14 is placed on a table 16 of a fixture indicated in
its entirety as 18. The bottom surface 14 is the one which is to be
chemically treated in accordance with the teachings of this
invention.
The table 16 is a disc shaped block having an outer periphery
generally corresponding to the periphery of the conventionally
circular wafer 10 and includes a relatively flat relatively
horizontal working surface 20, centrally apertured as at 22, for
supporting the wafer 10. The block is, in turn, supported on a
supporting plate 24.
The fixture 16 is divided into an upper liquid reclaim plenum
chamber 26 and a lower liquid plenum chamber 28 by dividers 30 and
32 and the lower chamber 28 is in open communication with the
aperture 22 in working surface 20 in any suitable manner, such as
by tube 34 externally threaded into internally threaded counter
bore 36 in the block 16. As shown in the drawings, the lower end of
the tube 34 is sealed as by threading at 38 into divider 30 to
prevent the liquid in the upper chamber from flowing into the lower
chamber. The liquid in the lower chamber 28, utilized to chemically
treat the lower surface 14 of the wafer 10, is pumped from the
lower chamber through the tubing 34 and the aperture 22 and thus
introduced between the two surfaces 20 of the block 16 and 14 of
the wafer 10. After spreading over the entire surfaces 20 and 14,
the liquid is allowed to drop around the outer edges of the block
16 and to pass through openings 40 in the support plate 24 where
the liquid is collected in the upper chamber 26. To recirculate the
liquid and to pump the liquid through the tube 34, the chamber 26
is provided with outlet 42 to which is attached a recirculating
pump 44 which pumps the fluid into inlet 46 of the lower chamber 28
under sufficient pressure to force the liquid up through the body
16 and introduce the same between the surfaces 20 and 14,
respectively. In order to regulate pressure exerted by the liquid
as it emerges from the aperture 22 and yet not disturb the general
orientation of the wafer 10 in relation to the surface 20, a
pressure regulating valve 48 is provided between the recirculating
pump 44 and the inlet 46. In addition, for anodizing purposes, a
suitable electrode 50 in the lower chamber 28 supplies the negative
potential to the liquid and a freely suspended electrode 52
supplies the positive potential to the wafer 10.
For the foregoing, it can be seen that a continuous flow of a
liquid chemical such as an anodizing solution, has been provided by
the apparatus of this invention and, while the physical phenomena
upon which this apparatus relies to perform is not entirely clear,
it is believed that it is a combination of gravity, surface
tension, and the Bernoulli effect. If this theory is correct, as
the liquid flows between the restricted passageway defined by the
wafer 10 and the surface 20, the velocity of the liquid increases
thereby creating a pressure drop between the surfaces with the
result that because of gravity, together with the atmospheric
pressure pressing down on the wafer, the wafer is maintained in
general orientation with the table surface yet will not slide off
the edge of the table due to fluid surface tension around the
periphery as long as is necessary for the chemicals to operate on
the surface 14. It has been found, for example, that with the rate
of flow is about 0.25 gallons per minute passing through the
aperture 22 of about 0.25 inches in diameter, over a top surface
within 3.degree.- 4.degree. of true horizontal, this fixture can be
satisfactorily utilized to anodize aluminum or a standard 3.0 inch
diameter silicon wafer with a 2% phosphoric acid anodizing
solution.
Turning now to FIG. 2, one can see that the method can be carried
out on a plurality of workpieces at the same time by simply
providing additional tables 16. It should be pointed out also in
this Figure that the plurality of electrodes 52 are shown connected
through the lid 54 of the fixture 12 and shown with pencil-like
tips 56 suspended over the wafer as compared to the schematic
showing in FIG. 1.
In connection with apparatus shown in FIG. 2 it should be pointed
out that to avoid placing the wafers to be treated individually on
each table, a large flat vacuum lid or table is utilized, though
not shown herein. This vacuum table has an indexing means spaced
thereon to locate each wafer corresponding to the location of the
corresponding table of the fixture 16. Wafers are placed on this
lid and held there by vacuum so that they can be placed face down
on the tables when the vacuum is released.
As hereinabove mentioned in certain applications, it has been found
that the liquid for chemically treating the undersurface 14 of the
wafer, while flowing out and over the edges of the table in certain
applications tends to creep over the outer edges of the wafer and
up on to the upper surface 12 particularly near the very edges of
the upper surface. In order to prevent this creeping phenomena, in
these instances the wafers are preprocessed by oxidizing the edges
by anodization in fixture such as shown in FIG. 3. For the same of
simplicity in describing the function of FIG. 3 those parts were
function is the same or similar to similar parts in FIGS. 1 and 2
will be given the same reference numbers but with a suffix a.
It is noted in FIG. 3 that the upper and lower plenum chambers are
in communication via a tube 34a with a Table 16a. However, in this
case the table 16a has been formed with an inner cavity 60 for
accommodating a vacuum table indicated in its entirety as the 62,
to form a chuck for holding the wafer in position relative to the
table 16a. The vacuum table is provided with a top surface 64 which
is slightly higher than the top surface 66 of the table 16a so as
to permit the liquid chemical from the lower chamber 28a to flow
out over the top surface 66 and back into the upper chamber 26a in
a manner similar to that described in connection with FIGS. 1 and
2. This liquid chemical from the lower chamber chemically treats
the outer edges 68 of the wafer as defined by the outer periphery
of the top surface 66 and the outer periphery of the vacuum table
64. In order to provide the suitable vacuum for the vacuum table in
order to hold the wafer thereon, the vacuum table 62 is provided
with a plurality of apertures 70 which are in open communication
through a inner conduit 72 to a vacuum chamber 74 which in this
embodiment, is located below the two chambers 26a and 28a and
connected to a suitable vacuum source (not shown). It should also
be noted that the physical phenomena relied upon to position the
wafer in the fixture of FIGS. 1 and 2 is not used in this
embodiment since the vacuum table is relied upon to hold the wafer
in position as the edges are being treated and it should also be
noted that the edges being treated will becom the top of upper side
12 of the wafer 10 as performed in the method and apparatus of
FIGS. 1 and 2. The wafer, preprocessed in the apparatus of FIG. 3,
is clearly shown in FIGS. 4 and 5 of the drawings with the area
identified as 76 showing the oxidized edges in exaggerated form for
purposes of clarity.
From the foregoing it can be seen that a new method has been shown
and described which will permit a workpiece, such as a
semiconductor wafer, to be chemically treated on one side only
without the necessity of a protective coating on the other side;
but in those applications where the creeping phenomenon is present,
and only if, this phenomenon is undesirable, a preprocessing step
can be provided in a simple manner. Thus, in connection with the
fisture of FIGS. 1 and 2 the procedure to form for example an
anodic oxide on the front or lower face of an aluminized wafer is
substantially as follows:
1. Load wafer on the tables with the surface to be turned face down
on the tables either individually by hand or by the use of a vacuum
lid on table,
2. Place electrical contacts touching the wafer backs (top of
wafers),
3. Start solution flow and apply desired voltage (5-1000) for
desired length of time (3 minutes-2 hours), and
4. Remove wafers, rinse and dry.
In those applications where preprocessing of the wafer is necessary
or desirable then the following steps would be taken:
1. Place wafers with the bottom faces down on the vacuum lid on
table.
2. Apply vacuum to hold the wafers.
3. Start solution flow and apply voltage (10-100 V) for desired
length of time.
4. Remove wafers, rinse and dry.
5. Start steps 1-4 of regular process above.
* * * * *